The present invention relates to the dewatering of wood byproducts to produce a furnish for a mechanical refiner or chemical digester, for conversion of fibers into pulp.
The purpose of a dewatering compression screw device in the overall processes of greatest interest, production of fiberboard (MDF), particleboard (PB), or thermomechanical pulp (TMP), is to remove fluids (extractives) from the raw material and form a pressure plug prior to entry of the dewatered material into the digester or refiner. The raw material is typically sawdust, wood shavings, or wood chips, which are generally considered bulk solids. No pre-processing is required before dewatering, except for washing or pre-steaming. In MDF or PB processes, an air dryer is typically provided after the refiner, and measurement of the heat or energy required for a target dryness of the refined material (dryer load), is an inverse measure of the effectiveness of the dewatering device upstream of the refiner. Even when no active drying is performed downstream of the dewatering device, the energy efficiency of the main process can be correlated with the effectiveness of the dewatering. In the TMP process, the extractives to be removed from the raw material source contribute to darkening of the final product. Removal of extractives may thus yield a reduction in the amount of bleaching chemicals necessary to produce the desired brightness.
For many years, efforts have been made to increase the dewatering effectiveness on the feed material, and thereby reduce the overall energy or chemical costs, especially for the final drying or bleaching stages. This has typically been characterized by reducing variations in the initial moisture content (arising for example, from storage conditions and seasonality), and increasing the compression ratio in the dewatering section of the screw device. Typical devices are as described in U.S. Pat. No. 5,320,034 and International publication WO 92/13710, the disclosures of which are hereby incorporated by reference. In this context, compression ratio (CR) is defined as the cross sectional area of the inlet versus the cross sectional area of the outlet of the dewatering device.
Dewatering of bulk solid materials has been particularly difficult. Whereas dewatering of pulp slurries having a consistency in the range of 4-5 percent is relatively easy, dewatering of bulk materials having a consistency greater than 25 percent, has posed problems. These problems arise from the difficulties in managing the relative friction among the screw shaft, screw flights, and compression housing wall. In particular, it is desirable that the friction in the axial direction be lower than the friction in the tangential direction along the wall, such that the material as influenced by the screw flights will be transported axially as well as compressed, rather than remaining between particular flights and rotating with the screw.
It is also known that when very high compression ratios are desired, single flight screws have inherent unbalanced loads and reach a point of diminishing returns whereby a given increment of applied energy (in the form of increased torque on the screw) produces very little benefit in the way of further dewatering. Thus, single flight screws are suitable for low consistency feed material, or only modest levels of compression of bulk solids. Double flight screws are known for use in higher compression environments, especially for bulk solids. Double flight screws, however, have required forced feeding, such as described in U.S. Pat. No. 5,320,034, thereby adding another drive device and increasing the total energy required for a given level of dewatering.